Wireless sensor networks are an emerging technology that is gaining popularity for its wide application. A typical wireless sensor network includes sensor nodes deployed in an ad hoc fashion for the purpose of physical sensing and monitoring of the environment. With wireless communication capability, these sensor nodes collaboratively disseminate the sensed data from a target location to a destination, which is usually a data gathering and processing point. Using this technology, users at the data gathering and processing point are able to monitor and probe a wide geographical area without being physically present at the target location.
FIG. 1 shows a wireless sensor network 10. The wireless sensors 12 are deployed in a perimeter fence. Intrusion by an intruder 16 that is detected 14 will be conveyed to the data gathering and processing point 24 and an alert may be triggered.
Wireless sensor networks suffer from serious energy constraints since the sensor nodes are powered by a battery. In Yujie Zhu and Raghupathy Sivakumar, “Challenges: Communication through Silence in Wireless Sensor Networks”, MobiCom '05, Aug. 28-Sep. 2, 2005, Cologne, Germany, an architecture uses packet timing to transmit information to conserve battery energy. In Zhu et al. a timer at the receiver node counts the data from the transmitter node by observing the packet timing.
FIG. 5 shows a timeline 30 for the transmitting from the transmitter node 32 and receiving at the receiver node 34 the control packets as described in Zhu et al. A start packet of two bits is transmitted 36 by the transmitter node 32 to initiate the timer at the receiver node 34 upon receipt 42 by the receiver node 34. Upon counting to the intended value 40 of n counts, the transmitter node 32 sends 38 a STOP packet of two bits to terminate the timer once the stop packet is received 44 by the receiver node 34. The numerical value counted by the timer is accepted as the transmitted information.
The communication methodology in Zhu et al. is referred to as virtual communication through delay response (VCDR). The primary motivation for VCDR is energy saving because no real information is physically transmitted. The power efficiency in VCDR is due to data suppression. By using merely start and stop packets, two wireless sensor nodes can exchange data without having to transmit the data bits physically. As power consumption is proportional to the length of data bits transmitted, the data suppression in VCDR reduces transmission and reception power.
Zhu et al. presents three optimization strategies and five challenges for VCDR.
The first optimization strategy is multiplexing. The silence time interval between the start and stop packets is used by another pair of nodes to communicate without causing interference. The second optimization strategy is cascading. The transmitter node has multiple numerical values to be sent to the receiver node and arranges the values in ascending order. The first communication session relays the lowest numerical value, and the subsequent communication sessions relay the higher numerical values by making the timer at the receiver node start counting from the previous counted value. This reduces the total time to relay the data. The third optimization strategy is fast-forwarding. The receiver node, upon receiving the start packet, immediately retransmits it to a downstream node without waiting for the stop packet. Similarly, upon receiving the stop packet, the receiver node retransmits it to the downstream node so that the same value is counted at the receiver and downstream nodes.
A challenge is framing in which the optimal length of data to be sent has to be determined. Shorter data leads to higher energy consumption due to more start and stop packets whereas longer data may introduce excessive delay. Another challenge is addressing. Adding a node unique address to the packet header increases overhead and lowers efficiency. Another challenge is sequencing. Adding a sequence number to the start packet increases the packet size. An additional challenge is contention control. Although the start and stop packets are relatively short, packet collision may occur. A further challenge is error control. New error control strategies are needed for VCDR.
VCDR has several critical drawbacks.
A first drawback of VCDR is time synchronization between the transmitter and receiver nodes. Delay variations between the start and stop packets due to transmission and propagation variables can easily corrupt the data and render VCDR impractical.
Another drawback of VCDR is the lack of error control techniques. Since VCDR is a relatively new and unknown communication technique, existing error control techniques are insufficient.
Still another drawback of VCDR is packet loss. For example, the loss of the stop packet can cause the receiver node to count excessively or indefinitely and in turn consume excessive energy.
There is thus a need for a method of communicating between nodes in an energy efficient manner in a network environment that alleviates the problems associated with current communication techniques.